Bioimpedance circumference measurement

ABSTRACT

Apparatus is disclosed for measuring the circumference of a limb of an individual, e.g., the individual&#39;s arm and/or calf. The apparatus uses one or more magnetic strips which surround the limb and contain magnetic coding of length information. Tension is applied to the magnetic strip by a tensioning assembly, which can be a pressure cuff or a stepping motor, and a magnetic read head reads the magnetic coding of length information from the strip. When used in a bioimpedance analysis procedure, the length information can be used to convert measured voltage differences into normalized bioimpedance values, e.g., resistivity values.

CROSS-REFERENCE TO RELATED CASES FOR U.S. NATIONAL PHASE APPLICATION

This application is a U.S. National Phase of International PatentApplication No. PCT/US2013/036921, filed on Apr. 17, 2013, which ishereby incorporated herein by reference in its entirety. Thisapplication is also a continuation-in-part of co-pending InternationalApplication No. PCT/US2011/55916, filed Oct. 12, 2011, which claims thebenefit under 35 U.S.C. § 119(e) of U.S. Provisional Application No.61/393,544, filed Oct. 15, 2010. This application also claims thebenefit under 35 U.S.C. § 119(e) of U.S. Provisional Application No.61/774,891, filed Mar. 8, 2013. The contents of InternationalApplication No. PCT/US11/55916 and U.S. Provisional Applications Nos.61/393,544 and 61/774,891 are hereby incorporated herein by reference intheir entireties.

FIELD

This disclosure relates to methods and apparatus for measuring thecircumference of a limb of an individual, e.g., an individual's calf.The measured circumference is used to determine a physiologicalproperty, e.g., the individual's hydration state, using a bioimpedanceprocedure. As one example, the methods and apparatus disclosed hereincan be used in determining the degree of fluid overload in dialysispatients.

BACKGROUND

Bioelectrical impedance analysis (BIA) is a commonly-used, non-invasivetechnique for estimating the composition of the body of a human oranimal. It has been practiced in whole body and segmental formats. Inbroad outline, current is applied to the body between at least twospatially-separated points (the current application points) and thevoltage difference produced by the applied current is measured betweenat least two other spatially-separated points (the measurement points).Typically, the measurement points are located inboard of the currentapplication points. Measurements can be performed at a single frequencyor at a series of frequency, in which case the technique is sometimesreferred to as BIA spectroscopy.

The impedance Z is determined by taking the ratio of the measuredvoltage V divided by the applied current I, where Z, V, and I are, ingeneral, complex numbers. Although the impedance Z can be of value forsome applications, normally, it is desirable to normalize the impedance(or one of its components) by the physical dimensions of the portion ofthe body over which the measurement was taken. For example, it is oftendesirable to derive a resistivity (ρ) value from a resistance (R) valueusing the equation ρ=R·A/L, where L is length and A is cross-sectionalarea, e.g., A=C²/4π for a circular cross-section whose circumferentiallength is C.

Of the two dimensions L and A, L is normally easier to estimate. Thus, Lcan be well-approximated by the linear distance between thespatially-separated measurement points. Estimating A, on the other hand,is more difficult for the fundamental reason that body tissues arecompressible.

Although health care and other professionals (e.g., weight loss coaches,physical trainers, and the like) can be taught to measure thecircumference of a portion of the body with a tape measure, themeasurement requires judgment as to how tight to make the tape. The needfor judgment results in substantial and unacceptable variability betweenmeasurements made by different professionals, as well as in measurementsmade by the same professional with different individuals or the sameindividual on different occasions. For lay personal, the problem ismarkedly worse. Moreover, other than for measurements on the legs,circumference measurements are difficult for an individual to do onhimself or herself, e.g., it is difficult to apply a tape measure toone's own arm. Even leg measurements can be difficult for someindividuals whose eyesight and/or dexterity has been compromised.

The present disclosure addresses this problem of unreliablecircumference measurement which has reduced the usefulness of BIA and,in particular, segmental BIA, both in clinical and at-home settings.

SUMMARY

Apparatus is disclosed for measuring the circumference of a limb of anindividual which comprises:

-   -   (a) one or more magnetic strips, each of which, during use of        the apparatus, surrounds the limb and each of which, along its        length, comprises magnetic coding of length information;    -   (b) a magnetic read head for each magnetic strip for reading the        magnetic coding of length information from the strip; and    -   (c) a tensioning assembly for setting the tension of the one or        more magnetic strips.

In accordance with an embodiment, apparatus is disclosed for measuringthe circumference of a limb of an individual comprising a pressure cufffor application to the limb, said pressure cuff comprising;

-   -   (a) one or more magnetic strips, each of which surrounds the        limb when the pressure cuff is applied to the limb and each of        which comprises magnetic coding along its length;    -   (b) a magnetic read head for each magnetic strip for reading        length information from the strip; and    -   (c) a plurality of air stripes for setting the level of tension        of the one or more magnetic strips.

In accordance with an embodiment, apparatus is disclosed for measuringthe circumference of a limb of an individual comprising a housing forapplication to the limb, said housing being handle-shaped and housing:

-   -   (a) one or more measuring tapes, each of which surrounds the        limb when the handle-shaped housing is applied to the limb, each        of which comprises a magnetic strip which has magnetic coding        along its length;    -   (b) a magnetic read head for each measuring tape for reading        length information from the magnetic strip; and    -   (c) for each measuring tape, a stepping motor for dispensing and        applying tension to the measuring tape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary embodiment of the presentdisclosure which employs a pressure cuff to apply current-injectingelectrodes, measuring electrodes, and at least one magnetic strip to thelimb of a subject in order to perform a bioimpedance procedure on thelimb.

FIG. 2 is a schematic diagram showing the cuff of FIG. 1 in more detail.

FIG. 3 is a schematic diagram illustrating one of the magnetic strips ofthe embodiment of FIGS. 1-2 and a magnetic read head for reading lengthinformation from the strip.

FIG. 4 is a schematic diagram of an exemplary embodiment of the presentdisclosure which employs a handle-shaped carrier to applycurrent-injecting electrodes, measuring electrodes, and at least onemagnetic strip to the limb of a subject in order to perform abioimpedance procedure on the limb.

FIG. 5 is a block diagram of exemplary system components for use in theembodiment of FIG. 4.

FIG. 6 is a schematic diagram showing an exemplary circumferencemeasuring system for use in the embodiment of FIG. 4 prior to deploymentof the system about a user's limb.

FIG. 7 is a schematic diagram showing the circumference measuring systemof FIG. 6 after deployment about a user's limb (not shown in thisfigure).

FIG. 8 is a graph comparing calf circumference measurements performedmanually by trained individuals (horizontal axis) with calfcircumference measurements performed using a commercial circumferencemeasuring device (vertical axis).

FIG. 9 is a Bland-Altman plot for the data of FIG. 8.

FIG. 10 is a photograph of the commercial circumference measuring deviceused in obtaining the vertical axis data of FIG. 8.

DESCRIPTION

FIGS. 1-3 show an embodiment of an integrated calf bioimpedance monitorwith circumference measurement. The apparatus uses one or more magneticstrips (see FIG. 3) integrated together with reusable or disposableelectrodes into a pressure cuff with multiple, separate, air stripeareas for applying pressure to the electrodes and for setting the levelof tension of the one or more magnetic strips during the circumferencemeasurement (see FIGS. 1 and 2).

FIG. 1 shows the device on a user's calf, and FIG. 2 shows the cuff ofFIG. 1 in more detail. Specifically, FIG. 2 shows the cuffs electrodes,tension sensors, air stripe areas and magnetic strips. In FIG. 2, theintegrated electrodes are shown at 1-4, the tension sensors at 8, 9, 10,18, and 19, the magnetic strips for obtaining circumference values at 7,12, and 16, and the magnetic read heads for reading the magnetic stripsat 13, 14, and 15.

Similar to a magnetic ID card, distance along the length of a magneticstrip can be scaled with, for example, 0.1 cm resolution by magneticcoding of the strip. The air stripe areas, e.g., the five areas as shownin FIG. 2, are inflatable separately with different pressures so as tocontrol the tension of each air stripe area.

The circumference can be calculated according to the values read by theone or more magnetic read heads (see FIG. 3), e.g., three magneticstrips and three magnetic read heads at, for example, three differentlocations along the length of the user's calf, e.g., at locations whichgive maximum, middle, and minimum circumference measurements (see FIG.1). Four electrodes, e.g., four reusable electrodes, are used, two forinjecting current and two for measuring voltage, e.g., for measuringvoltage between two measurement points 10 cm apart. The “input” and“output” components shown in FIG. 1 can, for example, be used to obtaindata such as body height and weight. The results can be stored in thedevice and can also be sent to a remote location using a cable or bywireless communication.

Among other applications, the device of FIGS. 1-3 can be used formeasuring body composition and fluid status for dialysis patients or anyother patients who need to check hydration status or changes in bodycomposition. The device can be used in a clinical setting or at home bya healthcare professional or by the person being measured by the device.

Among the advantages of devices of the type shown in FIGS. 1-3 are:

1) circumference can be measured for more than one cross-sectional area,e.g., three cross-sectional areas; 2) the interface between skin andelectrodes can be kept constant by pressure control; 3) the tensionsensors can be used to improve the accuracy of the circumferencemeasurement by ensuring that a desired level of tension (e.g., 100-150grams) is applied to the magnetic strips; 4) the multiple air stripeareas, e.g., the five air stripe areas shown in FIGS. 1 and 2, can beseparately inflated with variable pressure to take account of thevariety of geometric shapes of human and animal limbs, such as the calf;5) the electrodes can be disposable or reusable; 6) the device can beused to measure any body segment; and 7) when used on the arm, thedevice can automatically produce values of, for example, standard bloodpressure, degree of fluid status, muscle mass, and fat mass, with asingle measurement protocol.

FIGS. 4-7 show a further embodiment where the circumference measurementis performed using a handle-shaped device, rather than an inflatablecuff. The use of three circumference measuring tapes, i.e., threemagnetic strips, is shown in FIG. 4, it being understood that more orless strips can be used as desired. The “A view” in FIG. 4, i.e., themiddle panel, is a top view, and the “B view”, i.e., the lowest panel,is a bottom view. The uppermost panel is a side view. By means of thedevice's handle configuration, during use, the bottom of the device (Bview) can be easily pressed against the subject's skin, thus bringingthe current injecting and measuring electrodes into firm contact withthe limb whose properties are to be measured.

The major components and functions of the device of this embodiment areillustrated in the block diagram of FIG. 5. Two electrodes (E1 and E2)are used to inject current, e.g., current at different frequencies inthe range of, for example, 1 kHz to 300 kHz, and another two electrodes(E3 and E4) are used to measure the resulting voltage. In order toreduce noise, the signals can be pre-processed (e.g., with apre-amplifier and a low pass filter) before being used to calculate animpedance, a resistance, and/or a reactance value between the measuringelectrodes during the application of current between the injectingelectrodes. An analog-to-digital converter (A/D converter) and a digitalsignal processing unit (DSP unit) are used to convert the measuredanalog signal to a digital signal and then to calculate impedance,resistance, and/or reactance at the different applied frequencies.

Parameters, such as, body weight, height and gender of the user (e.g.,patient), can be inputted to the device using a small key board as shownin the middle panel of FIG. 4. A LCD display or other type of displaycan be used to display the inputted parameters and/or the results of thebioimpedance measurement and analysis, e.g., to display one or morecircumference values, such as a maximum circumference value C1 and aminimum circumference value C2, and one or more bioimpedance values,such as, a ρ value. Other possible outputs include a hydration index,fat mass, muscle mass, ECV, ICV, and the like. The device can include aUSB interface for downloading information from the device to a computeror communications device. As shown in FIG. 4, the device can include acontrol key for activating the bioimpedance measurement process.

FIGS. 6-7 show representative equipment for using a magnetic strip tomeasure the circumference of a user's limb. FIG. 6 shows a tape carryingthe strip in its stored configuration, while FIG. 7 shows the tapedeployed about the user's limb (not shown in FIG. 7). Specifically, 100is the tape with the magnetic strip, 200 and 400 are magnetic readheads, 300 is a stepping motor for dispensing and applying tension tothe tape, and 500 is a tension sensor.

For the embodiment of FIGS. 4-7, a gap exists between the circumferencemeasurement assembly and the user's skin, especially for the middle tapeshown in the top panel of FIG. 4. Such a gap can also exist for the sidetapes of FIG. 4, as well as for the embodiment of FIGS. 1-3, but usuallywill be of less concern. To address the potential effects of such a gap,a study was performed which compared manual calf measurements made bytrained personnel using a conventional tape measure (C_(Manual)) withmeasurements made with a commercial diameter measuring device (see FIG.10), where the user reads the length of the tape (C_(Device)) but thedevice determines the amount of tension applied to the tape.

The results from 49 subjects are plotted in FIG. 8, where C_(Manual) isplotted along the horizontal axis and C_(Device) along the verticalaxis. As can be seen in this figure, C_(Device) is highly correlatedwith C_(Manual) (R²=0.95). The Bland-Altman analysis of FIG. 9 shows adifference (bias) between C_(Manual) and C_(Device) of 1.0±0.74 cm. Thisbias can be considered a systematic error that can be easily compensatedfor when circumference lengths are being used to convert measuredvoltages into normalized bioimpedance values, e.g., resistivity values.The data of this experiment thus shows that measurement of calfcircumference using an automated device can provide accurate data torepresent the actual value of a limb's circumference, e.g., a calf scircumference.

The apparatus and methods disclosed herein can be used in a variety ofclinical applications, including without limitation: (1) measurement ofhydration state (degree of hydration) for CKD, dialysis, peritoneal, orhemodialysis patients; (2) measurement of nutrition state for allpatients through the provision of fat, muscle, cell mass, ECV, and/orICV values for the leg or arm; and (3) detection of bleeding at anylocation of the body for surgical patients during recovery through themeasurement of local changes in resistance.

It is to be understood that the foregoing summary and description ofexemplary embodiments is intended to provide an overview or frameworkfor understanding the nature and character of the invention. Additionalfeatures and advantages of the invention will be readily apparent tothose skilled in the art from that description or recognized bypracticing the invention as described herein. The accompanying drawingsprovide a further understanding of the invention, and are incorporatedin and constitute a part of this specification. It is to be understoodthat the various features of the invention disclosed in thisspecification and in the drawings can be used in any and allcombinations.

What is claimed is:
 1. An apparatus for measuring the circumference of alimb of an individual comprising: two or more magnetic strips, each ofwhich, during use of the apparatus, surrounds the limb and each ofwhich, along its length, comprises magnetic coding of lengthinformation; a magnetic read head for each magnetic strip for readingthe magnetic coding of length information from the strip; at least twocurrent-injecting electrodes for injecting current at a plurality offrequencies, and at least two voltage-measuring electrodes for measuringresulting voltage between the at least two current-injecting electrodes;at least one of a pre-amplifier and a low pass filter for processing asignal received from the at least two voltage-measuring electrodes; anA/D converter for converting the signal from the at least one of thepre-amplifier and the low-pass filter; a digital signal processing unitfor calculating at least one of impedance, resistance, and reactance atthe plurality of the frequencies of injected current based on theconverted signal received from the A/D converter, wherein the digitalsignal processing unit is configured to receive a single circumferencemeasurement determined from a plurality of circumference measurementsreceived from the magnetic strips; a tensioning assembly for setting thetension of the two or more magnetic strips; and a tension sensor foreach magnetic strip; wherein the apparatus is configured to measure thecircumference of the limb in a plurality of cross-sectional areassimultaneously for determining the plurality of circumferencemeasurements.
 2. The apparatus of claim 1 wherein the apparatus includestwo current-injecting electrodes and two voltage-measuring electrodes.3. The apparatus of claim 1 wherein the tensioning assembly includes apressure cuff.
 4. The apparatus of claim 3 wherein the pressure cuffincludes a plurality of inflatable air stripes.
 5. The apparatus ofclaim 4 wherein the inflatable air stripes are separately inflatable. 6.The apparatus of claim 3 wherein the pressure cuff includes twocurrent-injecting electrodes and two voltage-measuring electrodes. 7.The apparatus of claim 1 wherein the apparatus includes a handle-shapedhousing which has a middle portion and two side portions which extendaway from the middle portion, each side portion having a bottom whichcontacts the limb during use of the apparatus.
 8. The apparatus of claim7 wherein a magnetic strip is housed in the middle portion of thehandle-shaped housing.
 9. The apparatus of claim 7 wherein a magneticstrip is housed in each of the side portions of the handle-shapedhousing.
 10. The apparatus of claim 7 wherein a magnetic strip is housedin the middle portion and each of the side portions of the handle-shapedhousing.
 11. The apparatus of claim 7 wherein the bottom of each sideportion includes a current-injecting electrode and a voltage-measuringelectrode.
 12. The apparatus of claim 7 wherein the tensioning assemblyincludes a stepper motor.
 13. A method of measuring the circumference ofa limb comprising: applying an apparatus for measuring the circumferenceof the limb to the limb, the apparatus comprising: two or more magneticstrips, each of which, during use of the apparatus, surrounds the limband each of which, along its length, comprises magnetic coding of lengthinformation; a magnetic read head for each magnetic strip for readingthe magnetic coding of length information from the strip; at least twocurrent-injecting electrodes for injecting current at a plurality offrequencies, and at least two voltage-measuring electrodes for measuringresulting voltage between the at least two current-injecting electrodes;at least one of a pre-amplifier and a low pass filter for processing asignal received from the at least two voltage-measuring electrodes; anA/D converter for converting the signal from the at least one of thepre-amplifier and the low-pass filter; a digital signal processing unitfor calculating at least one of impedance, resistance, and reactance atthe plurality of the frequencies of injected current based on theconverted signal received from the A/D converter, wherein the digitalsignal processing unit is configured to receive a single circumferencemeasurement determined from a plurality of circumference measurementsreceived from the two or more magnetic strips; and a tensioning assemblyfor setting the tension of the two or more magnetic strips; wherein theapparatus is configured to measure the circumference of the limb in aplurality of cross-sectional areas simultaneously for determining theplurality of circumference measurements; activating the tensioningassembly; and reading length information from the magnetic strips. 14.The method of claim 13 wherein the limb is an arm.
 15. The method ofclaim 13 wherein the limb is a calf.
 16. A method of performing abioimpedance analysis comprising: applying an apparatus for measuring acircumference of a limb to the limb, the apparatus comprising: two ormore magnetic strips, each of which, during use of the apparatus,surrounds the limb and each of which, along its length, comprisesmagnetic coding of length information; a magnetic read head for eachmagnetic strip for reading the magnetic coding of length informationfrom the strip; at least two current-injecting electrodes for injectingcurrent at a plurality of frequencies, and at least twovoltage-measuring electrodes for measuring resulting voltage between theat least two current-injecting electrodes; at least one of apre-amplifier and a low pass filter for processing a signal receivedfrom the at least two voltage-measuring electrodes; an A/D converter forconverting the signal from the at least one of the pre-amplifier and thelow-pass filter; a digital signal processing unit for calculating atleast one of impedance, resistance, and reactance at the plurality ofthe frequencies of injected current based on the converted signalreceived from the A/D converter, wherein the digital signal processingunit is configured to receive a single circumference measurementdetermined from a plurality of circumference measurements received fromthe two or more magnetic strips; and a tensioning assembly for settingthe tension of the two or more magnetic strips, wherein the apparatus isconfigured to measure the circumference of the limb in a plurality ofcross-sectional areas simultaneously for determining the plurality ofcircumference measurements; activating the tensioning assembly; readinglength information from the magnetic strips; applying current to thelimb using the current-injecting electrodes; and measuring a voltagedifference between the voltage-measuring electrodes.
 17. The method ofclaim 16 further comprising: using the length information from themagnetic strips to convert a measured voltage difference into anormalized bioimpedance value.
 18. The method of claim 17 wherein thenormalized bioimpedance value is a resistivity value.
 19. The method ofclaim 16 wherein the limb is a calf.